Polyurethane foams are widely used in a variety of applications, including the packaging industry, in which polyurethane foams are used for cushioning fragile articles for shipping and handling. Various processes for producing polyurethane foams are known in the art. In general, a polyol-containing precursor and an isocyanate-containing precursor are brought together and mixed in the presence of a catalyst to cause a reaction which leads to curing and solidification of the mixture. A gas is introduced into the mixture prior to solidification so that foaming of the mixture occurs.
A desirable objective in mixing the precursors is to achieve sufficient mixing so that the resultant polyurethane foam is substantially uniform and has the desired density for the intended application. Chemical and/or mechanical mixing techniques have been used for aiding the mixing of the polyol and isocyanate precursors. For instance, chemical blowing agents such as hydrocarbons, fluorocarbons, chlorofluorocarbons, and the like, have been used for introducing gas into the precursors to promote a foaming action which also facilitates mixing of the components. However, such chemical agents are costly, and some pose environmental and health hazards.
Consequently, mechanical mixing techniques have been developed for introducing gas into the precursors and for mixing the two precursors. For example, U.S. Pat. No. 5,472,990, issued to Craig et al., describes a method of producing polyurethane foam in which a polyol precursor is mixed with air in a dynamic mixer, and the polyol/air compound is then mixed with an isocyanate precursor in a static mixer. The mixture is discharged from the exit of the static mixer into the workpiece or site where polyurethane foam is needed.
Processes such as the one described in the Craig patent are effective for producing uniform polyurethane foams. However, a major drawback to all such processes in which the two precursors are mixed within a through-flow device, such as a dynamic or static mixer, is that polyurethane begins to form instantly inside the device as the two components mix and begin to react. As a result, unless steps are taken to prevent build-up of polyurethane within the mixing device, the device will eventually become clogged and will cease to function properly. This build-up of polyurethane is conventionally overcome by frequent maintenance of the mixing devices to keep them unclogged and working, including flushing of the devices with a solvent to dissolve the polyurethane deposits from the insides of the devices. However, these maintenance and flushing procedures take time away from more-productive activities. Additionally, the flushing systems add electromechanical complexity and, consequently, add to the cost of a dispensing system with no corresponding gain in efficiency or usefulness. Furthermore, the solvents are costly, and their use and disposal can pose health and environmental problems.
Thus, there has been a need for a method of producing polyurethane foam in which hazardous mixing agents and/or solvents are not required, and which alleviates the problem of mixing devices being clogged by polyurethane deposits.